Project Summary/Abstract Bipolar disorder (BD) and other depressive disorders are some of the most common mental illnesses affecting millions of people. The primary treatment, lithium (Li+), has remained unchanged for > 50 years. However, the therapeutic window for Li+ is very narrow (0.5-1.2 mM) and the side effects can be severe. Also, Na+ dysregulation is implicated in BD and its pathways are also involved in Li+ uptake, but very little is known about the basis of BD or the mechanism of action of Li+. A critical barrier to maximizing the therapeutic effect and minimizing the side effect is missing information of the concentrations and distributions of Li+ and Na+ in BD cells, because of lack of selective sensors for Li+ and Na+ in living cells. This R21 proposal seeks to explore and demonstrate proof-of-concept of a novel class of DNAzyme sensors with high specificity for either Li+ or Na+ for simultaneous detection of Li+ and Na+ in BD cells, in order to provide new insights into the cellular and molecular mechanisms of Li+ treatment of BD. Specifically, we propose to use an in vitro selection method to obtain Li+- or Na+-specific DNAzymes with high catalytic activity towards ribonucleotide cleavage. These sequences will be characterized and optimized for sensor applications, whereupon we will transform them into fluorescent sensors using the catalytic beacon method for simultaneous imaging of Li+ and Na+ in lymphoblast cell models of BD, including cells obtained from BD- and non-BD patients. These fluorescent sensors will further be improved upon by incorporating both a photolabile-caging group to protect against ribonucleotide cleavage during the cell transfection, and a FRET pair of fluorophores to enable ratiometric sensing to allow for better quantitation of ion concentrations. While most cellular sensors have been designed for detection of divalent metal ions (e.g. Ca2+), few effective cellular sensors have been developed for monovalent ions. By developing a novel class of DNAzyme sensors to provide a direct measure of Li+ and Na+ simultaneously in BD cells, this proposal provides a critical missing piece of information in providing insights into the cellular and molecular mechanisms for Li+ treatment of BD. In the process, we will demonstrate general methodologies for DNAzyme-based sensors applicable for any metal ion, including the use of in vitro selection incorporating negative selection to improve selectivity and use of the catalytic beacon system to transform metal binding into different fluorescence readouts. The methods demonstrated can be applied for detection and imaging of Li+ and Na+ for other BD cells and can be generalized to develop similar sensors to image many other metal ions, which will advance the fields of mental health, cell biology, and clinical diagnostics.